Field of the Invention and Background
This invention relates to instruments for the noninvasive quantitative measurement of constituents in blood, such as blood glucose levels. Specifically, this invention relates to an improved analysis instrument utilizing an improved measurement of the temperature of the body part, typically a finger, which is being irradiated with near-infrared energy for measurement of the blood analyte levels.
Information concerning the chemical composition of blood is widely used to assess the health characteristics of both people and animals. For example, analysis of the glucose content of blood provides an indication of the current status of metabolism. Blood analysis, by the detection of above or below normal levels of various substances, also provides a direct indication of the presence of certain types of diseases and dysfunctions. In U.S. Pat. No. 5,077,476, incorporated by reference herein, it is taught that noninvasive blood glucose measurement can be made by transmitting near-infrared light through an extremity of the body, e.g., the most distal portion of the index finger. The temperature of the body at the point of measurement can be an important parameter because the peak wavelengths for near-infrared light passing through water (the human body is approximately 67% water) shift as a function of temperature. Thus, a means to insure that the shift in peak wavelength as a function of temperature does not interfere with the analyte measurement is important.
In the aforementioned '476 patent, the finger temperature can be taken into account as a separate term in a linear regression algorithm used in a near-infrared measurement instrument such as represented by the following equation: EQU C=K.sub.0 +K.sub.1 [log 1/I.sub.A -log 1/I.sub.B ]/[log 1/I.sub.D -log 1/I.sub.E ]+K.sub.2 T.sub.S +K.sub.3 T.sub.A
wherein C is the analyte concentration, K.sub.0 through K.sub.3 are calibration constants, T.sub.S is the local surface temperature of the finger and T.sub.A is the temperature of the air within the instrument, and the log 1/I terms represent optical density values at particular near-infrared wavelengths A, B, D and E. The calibration constants can be generated by various regression techniques such as multiple linear regression, stepwise regression, partial least squares fitting, principle component analysis, etc.
FIG. 1 illustrates a near-infrared blood analyte measurement instrument as disclosed in the '476 patent. Briefly, up to six or more IREDs (Infrared Emitting Diodes) represented in the figure by IREDs 5 and 6, detector 8 and processing means 10 are contained within a lightweight hand-held housing unit 1. Illustrative IREDs 5 and 6 are separated by light baffle 4 and are positioned so that the near-IR energy is directed through window 14, which may be light scattering, and onto the skin of the test subject. Optical filters, illustrated at 16 and 17, are positioned between each IRED and the window 14 for filtering the near-IR energy, thereby optimizing the band of near-IR energy striking the subject. It is very important that the test subject's fingertip not be exposed to ambient light. Further, it is desirable that the actual measurement be made near the rear of the finger nail. Finger stop 30 illustrated in FIG. 1 facilitates properly positioning the test subject's finger. FIG. 1 also illustrates a finger retainer 2 to securely position the user's finger inside the instrument and to provide sufficient blockage of ambient light. Spring 21 pushes the finger retainer 2 against the bottom of the test subject's finger thereby providing a secure fit. Linear potentiometer 19 is connected to finger retainer 2 and can measure an individual's finger thickness. In addition, an inflatable diaphragm or rubber/foam iris (illustrated at 15) is used to secure the test subject's finger and shield light.
FIG. 1 illustrates that when the finger is inserted into chamber 28, a built-in thermistor 29 measures the finger's temperature. A second thermistor 27 is positioned inside the analytical instrument 1 for measuring the ambient air temperature therein. The ambient air temperature measurement could be made at any time prior to the instrument's actual use, but preferably at the time the optical standard is measured. As shown in FIG. 1, the thermistor 29 is disposed near the optical measurement point of detector 8. While such an approach may be feasible, it raises complex engineering problems in that the location for the temperature measuring device is very close to the optical measurement path of the optical sensor. The resultant crowding of these two separate measurement devices can cause errors in either or both of the optical measurement and the temperature measurement.